What is meant by lap length of reinforcement and why is it required?

Lap length is the minimum length of overlap required between two serial rebars placed in concrete to transfer tension or compression from one bar to the other. For M25 concrete and Fe415 steel, it is approxiamtely 45d in tension and 35d in tension. The purpose of this length is to allow the transfer of force from one bar to concrete through shear (skin friction) and from concrete to the other bar through shear. If the length is insufficient, only a small force will go through shear and the rest will cause tension (or compression) in concrete, resulting in its failure.